(1) Field of the Invention
This invention relates to a process for the preparation of moulded polyolefin polymer products, in particular to the moulding of a particulate polymer material by rotational moulding techniques and to the particulate polymer material and the moulded polymer products.
(2) Description of the Related Art
Rotational moulding is a polymer moulding technique which is particularly suitable for the production of large polymer products, especially containers. It is quite different from other conventional moulding techniques such as injection moulding or blow moulding. A mould is charged with polymer powder, closed and placed in an oven where it is rotated so as to distribute the polymer powder over the mould surface. Once the polymer has melted and formed a coating on the mould surface the mould is cooled. Rotational moulding is described for example by Oliveira et al. in J. Materials Sci, 31: 2227-2240 (1996), Bawiskar et al in Polymer Engineering and Science 34: 815-820 (1994) and Bruins, xe2x80x9cBasic Principles of Rotational Mouldingxe2x80x9d, Gordon and Breach, NY, 1971.
The polyolefin polymer powder used in rotational moulding, e.g. a polypropylene or more generally a polyethylene, requires the presence of stabilizers, including UV-stabilizers, to prevent degradation between the time the polymer is produced and when it is moulded. This is normally achieved by mixing polymer and stabilizers in an extruder mixer which applies shear force to mix the components and,melt the polymer. The extrudate is then ground to produce a moulding powder of appropriate particle size. Such a procedure however is highly energy-consuming.
An alternative way of producing the stabilized moulding powder might thus have seemed to be to simply blend the stabilizers with an olefin polymer particulate which already has the appropriate particle size for rotational moulding, e.g. by spraying of liquid stabilizers or stabilizer solutions onto the polymer particulate and/or by simply mixing particulate stabilizers into the polymer particulate. This however results in unacceptable deposits of the UV-stabilizer on the surface of the mould-used in rotational moulding.
It has now been found that the moulding powder for rotational moulding may be sufficiently stabilized by simple mixing of a polyolefin polymer powder with a small quantity of a masterbatch of UV-stabilized-loaded polymer particles. In this way polyolefin powder particles as produced in a polymerization reactor may be used directly to produce the moulding powder without requiring energy-intensive extruder mixing and granulation and grinding of the entire material used to produce the moulding powder. Furthermore the problem of deposits of UV-stabilizer on the mould is reduced or avoided.
Thus viewed from one aspect the invention provides a process for the preparation of a moulded polymer item, said process comprising obtaining a polymer moulding powder having a mean particle size of 1 to 2000 /xcexcm comprising an admixture of UV-stabilizer-loaded polymer particles and non-UV-stabilizer-loaded polyolefin polymer particles, loading said powder into a mould, heating and rotating said mould to melt said powder, cooling said mould, and releasing said moulded polymer item from said mould.